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Digital Modulation and Demodulation

8. Digital Modulation and Demodulation. Digital Modulation Techniques. Digital Modulation Modifying amplitude, frequency, or phase characteristics of carrier with data stream representing ones and zeroes of computer language. Simplest schemes change carrier state with each bit of data.

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Digital Modulation and Demodulation

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  1. 8 Digital Modulation and Demodulation

  2. Digital Modulation Techniques • Digital Modulation • Modifying amplitude, frequency, or phase characteristics of carrier with data stream representing ones and zeroes of computer language. • Simplest schemes change carrier state with each bit of data. • Amplitude-shift keying (ASK). • Phase-shift keying (PSK).

  3. Digital Modulation Techniques • Amplitude-Shift Keying • “Keying” holdover from Morse code era. • Continuous wave (CW) • Carrier conveying intelligence by simply turning it on or off according to prearranged code. • Two-tone modulation • Form of AM, but in it carrier always transmitted.

  4. Digital Modulation Techniques • Frequency-shift Keying • Frequency modulation • Modulating wave shifts output between two predetermined frequencies. • Generation of FSK accomplished by switching additional capacitor into tank circuit of oscillator when transmitter keyed.

  5. Digital Modulation Techniques • Phase-shift Keying • Transmitted binary data cause phase of sine-wave carrier to shift by defined amount.

  6. Digital Modulation Techniques • Synchronization and Carrier Reinsertion • Carrier synchronizer • Producing reference carrier signal for product detector. • PLL • Heart of carrier synchronizer; used for all forms of digital modulation.

  7. Digital Modulation Techniques • Differential Phase-shift Keying (DPSK) • DPSK uses BPSK vector relationship for generating an output. • DPSK receiver simpler to implement. • Does not require coherent carrier recovery subsystem.

  8. Digital Modulation Techniques • Minimum Shift Keying • Widely used variation on FSK/PSK; used in cordless phones. • Continuous-phase FSK • Periods of mark and space frequencies such that cycles of sine-wave carrier cross zero right at edges of modulating-signal pulse transitions. • Minimizes bandwidth by ensuring smooth transition between data states.

  9. Bandwidth Considerations of Modulated Signals • Bandwidth of channel will not be infinite. • Transmitted pulse not perfect rectangle. • Bandwidth requirements for data stream depend on encoding method used to transmit data. • Sidebands • Sum and difference frequencies.

  10. Bandwidth Considerations of Modulated Signals • Bandwidth of modulated signal twice that of highest modulating signal frequency. • Symbol • Grouping of two or more bits. • Baud rate • Symbol rate.

  11. M-Ary Modulation Techniques • M-Ary PSK • Employs two data channels; one phase-shifted by 90° with respect to the other. • Non-phase-shifted channel • I (for “in-phase”). • Phase-shifted channel • Q (for “quadrature”).

  12. M-Ary Modulation Techniques • M-Ary PSK • Each channel contributes to direction of vector within phase constellation. • QPSK transmissions • Data bandwidth compression.

  13. M-Ary Modulation Techniques • M-Ary PSK • QPSK and forms of M-Ary modulation carried out with DSP-based chipsets. • I and Q bit streams considered two independent components of modulated signal that, when combined, form composite signal.

  14. M-Ary Modulation Techniques • Quadrature Amplitude Modulation • QPSK technique can be extended to more than four phase states. • If eight states, each point on constellation represents 3-bit binary symbol. • Constellation pattern visual indication of system linearity and noise performance.

  15. M-Ary Modulation Techniques • Offset Modulation • Because QAM schemes based in part on amplitude modulation, they suffer from drawbacks associated with any form of AM.

  16. Spectral Efficiency, Noise Performance, and Filtering • Spectral Efficiency • Combining bits into multiple-bit symbols improves bandwidth efficiency. • Bandwidth efficiency • Measure of how well modulation scheme handles data within defined bandwidth. • See Table 8-2: Spectral Efficiencies of Digital Modulation Formats

  17. Table 8-2 Spectral Efficiencies of Digital Modulation Formats

  18. Spectral Efficiency, Noise Performance, and Filtering • Noise Performance • Modulation methods having fewest symbols or lower spectral efficiencies have best performance in presence of noise. • Bit error rate (BER) • Signal-to-noise (S/N) ratio • Carrier-to-noise (C/N) ratio

  19. Spectral Efficiency, Noise Performance, and Filtering • Filtering • Goal • Reduce transmitted bandwidth without removing content of data to be transmitted.

  20. The Complex Exponential and Analytic Signals • Complex Numbers • Impedance • AC quantity (Z) composed of both resistance (R) and reactance (X). • Reactive components 90° out of phase with resistive component. • Complex operations • Arithmetic operations involving both real and imaginary quantities.

  21. The Complex Exponential and Analytic Signals • Analytic Frequency • Concept abstract; useful in how digital modulation and demodulation carried out in DSP.

  22. The Complex Exponential and Analytic Signals • DSP Modulation and Demodulation • Modulation of I/Q signal • Process of frequency up-conversion; moving baseband frequency range to higher frequency range of carrier. • Baseband signal both positive and negative frequency components. • Implementations of modulators and demodulators as DSP functions avoid all problems of analog circuitry.

  23. Wideband Modulation • Spread-Spectrum Techniques • Occupied bandwidth deliberately made to be much greater than specified by Hartley’s law. • Frequency hopping and direct sequence • Spreading out transmitted energy over wide bandwidth.

  24. Wideband Modulation • Spread-Spectrum Techniques • Cellular telephones, mobile Internet, wireless local-area networks (WLANs), automated data collection systems using portable scanners of universal product code (UPC) codes.

  25. Wideband Modulation • Spread-Spectrum Techniques • Based on stream of data bits displaying pseudonoise (PN) characteristics. • Spread-spectrum circuits have PN sequence generators integrated into system IC.

  26. Wideband Modulation • Spread-Spectrum Techniques • Frequency-hopping spread spectrum • Information signal modulates carrier switched in frequency in pseudorandom fashion. • Direct-sequence spread spectrum (DSSS) • Basis for cellular systems employing CDMA standard.

  27. Wideband Modulation • Orthogonal Frequency-Division Multiplexing (OFDM) • Multi-carrier (multi-tone) modulation. • Transmits data at slow rates over multiple carriers simultaneously; achieves high spectral efficiencies. • Information divided into multiple data streams.

  28. Wideband Modulation • Orthogonal Frequency-Division Multiplexing (OFDM) • OFDM modulation converse of demodulation. • HD radio technology • In-band on-channel (IBOC). • Flash OFDM • Spread-spectrum technology.

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